In a carriage printer, such as an inkjet carriage printer, a printhead is mounted in a carriage that is moved back and forth across the region of printing. To print an image on a sheet of paper (sometimes generically referred to as print medium or recording medium herein), the paper is advanced a given nominal distance along a media advance direction and then stopped. Paper advance is typically done by a roller and the nominal distance is typically monitored indirectly by a rotary encoder. While the paper is stopped and supported on a platen, the printhead carriage is moved in a direction that is substantially perpendicular to the media advance direction as marks are controllably made by marking elements on the paper—for example by ejecting drops from an inkjet printhead. Position of the carriage and the printhead relative to the print medium is precisely monitored, typically using a linear encoder. After the carriage has printed a swath of the image while traversing the paper, the paper is advanced, the carriage direction of motion is reversed, and the image is formed swath by swath.
In order to produce high quality images, it is helpful to provide information to the printer controller electronics regarding the printing side of the recording medium, which can include whether it is a glossy or matte-finish paper. Such information can be used to select a print mode that will provide an optimal amount of ink in an optimal number of printing passes in order to provide a high quality image on the identified media type. It is well-known to provide identifying marks or indicia, such as a bar code, on a non-printing side of the recording medium to distinguish different types of recording media. It is also well known to use a sensor in the printer to scan the indicia and thereby identify the recording medium and provide that information to the printer control electronics. U.S. Pat. No. 7,120,272, for example, includes a sensor that makes sequential spatial measurements of a moving media that contains repeated indicia to determine a repeat frequency and repeat distance of the indicia. The repeat distance is then compared against known values to determine the type of media present.
Co-pending US Patent Application Publication 2009/0231403 discloses the use of a backside media sensor to read a manufacturer's code for identifying media type. In this approach light from a light source is reflected from the backside of the media and received in a photosensor while the print media is being advanced past the photosensor. A source of unreliability in interpreting the signals is that media can slip during advance past the photosensor.
Co-pending US Patent Application Publication 2010/0149246 discloses reflecting light from a surface such that the reflected light is sensed by a sensor. In this system, one of the optical components is mounted to a movable device. As in US Patent Application Publication 2009/0231403 described above, in order to detect a manufacturer's code for identifying media type, the light is reflected from the backside of the media. Such an approach is compatible with media travel paths in which the backside of the media is viewable. However, this is difficult in some other types of media travel paths, especially where the printing side of the media faces outward away from the stack of media throughout the entire travel path.
Identification of media type by using transmitted light to detect a manufacturer's code, such as a bar code, has been disclosed in US Patent Application Publication 2006/0044577. In this application, the media is advanced past a transmissive sensor assembly including a light source and a transmissive optical sensor. As in co-pending US Patent Application Publication 2009/0231403, a source of unreliability in interpreting the signals is that media can slip during advance past the optical sensor.
Other disclosed approaches use both reflection and transmission of light simultaneously in the same printer to detect the media type. For example, U.S. Pat. No. 6,960,777 B2 positions a first light source on one side of the media and a second light source on the opposite side of the media with a sensor also positioned on the second side. The sensor receives light transmitted through the media from the first light source, and reflected light from the second light source. A ratio of the received reflected and transmitted light is then used to determine the media type.
Another prior art system, U.S. Pat. No. 7,015,474 B2, also uses both reflection and transmission of light simultaneously. This system positions a light source and a first sensor on a first side of the media, and a second sensor is positioned on the second side. The first sensor receives reflected light and the second sensor receives transmitted light both of which are used to determine a characteristic of the media.
Although these prior art systems are satisfactory, they include drawbacks. For example, using a ratio of reflected light to transmitted light includes the drawback of not compensating for the degradation of devices over time which will cause the ratio to deviate from expected results. Furthermore, systems which rely on moving the media past a sensor in order to read a manufacturer's code can be adversely affected in detection of sizes or distances between features of a manufacture's code if the media slips relative to the roller whose rotation is monitored, for example, by a rotary encoder. In other words, the position of the media is only indirectly monitored. Although the position of the roller can be well known, the position of the media can vary in unexpected ways relative to the roller.
Co-pending U.S. patent applications (dockets Ser. Nos. 12/604,428, 12/604,434 and 12/604,447) disclose overcoming these drawbacks by using a carriage-mounted sensor, whose position relative to the print medium is directly monitored, and by using light transmitted through the print media from a light source having a field of illumination that extends across the region where the manufacturer's code on the media will be located. As disclosed in those applications, although a single large light source can be used to provide illumination, one or more smaller light sources can be advantageous in that they can be compactly fit into the platen which supports the print medium in the region across which the carriage passes. Because the light from a small light source falls off in intensity as it spreads out further from the light source, it can be advantageous to have a plurality of light sources. In order to reduce cost, it is desirable to have relatively few light sources. However, if the light sources are spread out at too large of a spacing, the composite field of illumination becomes spatially nonuniform to an extent that can compromise the reliability of reading manufacturer's codes accurately.
What is needed is a method of providing substantially uniform illumination to a field of view of a light sensing device that is moving with respect to a light source or light sources having a spatially nonuniform field of illumination.